1. Field of the Invention
The invention refers to a device comprising a plate with at least two clearance holes for screws, and screws for fastening said plate to a bone, each screw including a threaded portion to be screwed into the bone and retaining means for retaining the plate.
2. Description of the Prior Art
Devices for immobilizing bone pieces and comprising plates to be screwed into the pieces of broken bone with screws have been in use in bone surgery for a long time. Attention is called in this connection to U.S. Pat. Nos. 3,552,389 and 4,219,015. The screws of this known device comprise a threaded portion and screw heads having approximately hemispherical bearing surfaces. The plates are provided with slots serving as clearance holes for the screws. Certain ones of the slots may comprise countersunk portions for receiving the screw heads and designed so, that when tightening the respective screw the plate becomes displaced in a particular direction.
If in the course of a surgical operation two pieces of bone are required to be connected with each other, it is customary to bend a plate to the proper shape and fasten it to the bone pieces, so that pressure contact is achieved between the contact surfaces. Experience has shown, however, that frictional corrosion may occur in those places in which the screws engage the plate. This happens in particular if time variable loads acting on the bone or the bone pieces are large enough to cause sections of the plate to perform micromovements, i.e. small movements, in relation to the screws and the bone pieces, in spite of the static compression force generated by the tightening process. These micromovements can also cause a reabsorption of bone material at the bearing surface of the plate. This reabsorption may cause open gaps to form between the plate and the bone pieces. The plate may then move back and forth between the screw head and the bone pieces. This may cause, under certain circumstances, a loosening of the threaded portions of the screw screwed into the bone pieces. The connection between the plate and the bone pieces continues to loosen more and more, so that the plate stops fulfilling its function of connecting the bone pieces with each other.
Mention is made here of the fact, that many different screw locking devices are known in the art. The Swiss Pat. No. 106 842, for example, discloses a screw locking device having a purpose not described in detail, but not intended for fastening implants. The screw includes a slot having a wedge inserted into it. A wood screw is disclosed, among other things, provided with a conical screw head and a slot passing through one portion of the screw shaft. A metal wedge is driven into the slot and into the material of the screwfastened portion surrounding the slot. This wedge is formed by a small plate essentially plane and provided at its forward edge with a wedge-shaped knife edge.
The Swiss Pat. No. 106 842 gives no indication to the effect, that the screw locking devices disclosed therein are intended to be used for fastening implants. The screw locking device, which comprises a wedge protruding sideways beyond the screw as disclosed in the Swiss Pat. No. 106 842 would in fact be unsuited for fastening implants, because the wedge would have to be driven into the bone plate and possibly into the bone too. However, the wedge could only be driven into the bone plate, made for example of steel, if the plate had been initially provided with slots for receiving the wedge. However, slots of this kind would cause a considerable weakening of the plate and, in addition, it would allow applying the wedge in certain definite positions of rotation of the screw only.
The Swiss Pat. No. 569 202 also discloses a screw locking device comprising a screw with a head and a threaded portion, the latter being provided with slots at its end. The screw possesses a longitudinal opening with a conical portion located in the region of the slots and a threaded portion extending in continuation of the conical portion. An expander having a conical head, a thread and a thinner shaft is screwed into the longitudinal opening. The Swiss Pat. No. 569 202 too, fails to give any indication relative to the possibility of using the screw locking device disclosed therein for surgical purposes. Since the screw locking device disclosed in Swiss Pat. No. 569 202 is subject to expansion at the free end of the threaded portion of the screw, this device too, would offer no improvement in the connection between the screw and the bone plate.
The French Offenlegungsschrift No. 2 193 161 refers to the riveting and screw-fastening of sheet metal plates. A bushing provided with a conical inner surface is first inserted in each of the bores of the plates to be connected with each other. In one illustrated embodiment, a screw having a conical neck is then inserted in a manner to produce locking, subsequent to tightening the screw. Mention is made in this connection of the fact, that the bushing could be provided with a slot extending over its entire length. The French Offenlegungsschrift thus refers to the screw-fastening of sheet metal plates and does not contain anything that would indicate that the connection disclosed therein could be adapted to the fastening of bone plates.